This will be made by one of the Yponomeuta spp., the group (at least the spotty ones) are known as Small Ermines. There are 8 UK species but several are very similar and it is likely that there is a complex of more species than currently described. Hence the adults can be very tricky to ID but as larvae you are aided by the foodplant. They live gregariously and make these impressive tracts of webbing to protect themselves from predators. Could this row of shrubs be Blackthorn? If so Yponomeuta padella is likely to be your man - or caterpillar.

Thank you for this information - also from all who replied. I think, looking back, that there was a lot of blackthorn around the area. Having never seen such an impressive web in this area, or anywhere, I was amazed. There was also a smaller, darker, webbing around lower growing plants but picture did not come out well enough to indentify.